H. Koops
Institut für Küsten-und Binnenfischerei
Bundesforschungsanstalt für Fischerei
Hamburg, Federal Republic of Germany
Freshwater eels of the genus Anguilla are represented by two species in the North Atlantic region and more than a dozen species in the Indo-Pacific region. The two Atlantic species, Anguilla anguilla and A. rostrata, the Japanese eel, A. japonica, and A. australis from New Zealand are of major importance in fisheries. All Anguilla species are similar in appearance and behaviour. Thus, the following remarks, which mainly concern the European eel, may apply more or less to all species. As eels differ so much in their life history from all other species of freshwater fish, a special chapter on methods for their sampling is justified.
If we use the strict meaning of the word “stock”, for all specimens originating from or belonging to one spawning community, each species of Anguilla most probably is represented by only one “stock”. A unique spawning area is known for the European eel and thus all eels from this continent as well as from North Africa belong to one “stock”, although investigations by J. and I. Boetius (pers.comm.) indicate that some A. rostrata occur among glass eels. This means that no sample can be representative for the “stock” in all respects, except a sample of eggs and larvae in the Ocean and perhaps of mature eels at or near the spawning area. Every sample will be representative only for species-related characteristics, which are not affected by the environment (as for example, perhaps special amino-acid or enzyme patterns of tissues and some meristic parameters).
Samples in fisheries are commonly used for studies of population dynamics, stock assessment, natural and fisheries mortality, etc. No single sample available between larval stage and spawners, and these are normally the only samples we can get, is valid at the “stock” level.
All species of Anguilla spawn in the sea, have floating eggs, and planktonic leaf-like larvae which metamorphose to transparent glass eels near the coast. The glass eels migrate up streams into fresh waters, where they grow to adults, and return to sea for spawning.
The sampling of eggs and larvae is relatively easy as this stage can be found in plankton catches. However, there are difficulties with identification and also knowing where, when and at what depths the catch occurred. This type of sampling is also restricted by costs and the effort involved, so our present knowledge on Anguilla anguilla is based mainly on the well-known investigations and publications by Schmidt (1925). Further activities are scheduled for the near future on an international basis, when attempts will also be made to catch spawners which should be pelagic and thus caught by midwater trawls. Again, the only problems will be the time, area, depth and density of sample fishings and consequently the money and effort needed not only to catch the fish, but to get catches large enough and stratified enough for “stock” size estimates.
In fresh inland waters including coastal, salt and brackish water areas, it is practicable to use the term “stock” for all eels living in a certain water body whether a lake or river or any part of such a system, assuming that there is little or no exchange with eels from other areas. We may thus talk about the eel stock of, for example, the River Shannon or the Shannon system as a whole, or Lough Dergh as one of the Shannon lakes. Such a water body may have a natural eel stock open to immigration of young eels, or the stock can be maintained artificially in the case of landlocked lakes, reservoirs lacking fish or eel passes, by which to circumvent the dam, rivers outside the natural distribution of elvers such as the Danube. In some other water bodies the eels can originate from minor natural immigration as well as from artificial stocking.
When there is artificial stocking, the stocking material, which consists of glass eels or pigmented elvers of several year-classes, can be easily sampled for such parameters as length, weight, age distribution, etc., although sub-samples may vary considerably even if taken from densely crowed tanks.
Upstream-migrating glass eels are surface-swimming during the first weeks after entering fresh water and when leaving the tidal zone they tend to swim in a band-like formation near the banks. They can easily be caught by means of dipnets from the banks or surface-drawn “minitrawls” of adequate mesh size from boats at this time. Frequency, time, distance or area of catches can be correlated with time of day, moon phase or tide, in order to obtain relative estimates of abundance. Comparisons by day, week or year can also be made. If connected with direct observation, these catches may give estimates of the total quantity immigrating per time (daily or annually).
In some countries (e.g., France, England, Ireland, Germany and Japan) there are fisheries for surface-migrating elvers. Favourable places are locations where the migration is blocked or hindered by natural or artificial obstructions (water falls, dams), so that the elvers concentrate in small areas. Commercial catches in such situations can also be sampled.
After some time, the glass eel becomes pigmentated and adopts a bottom-living habit while still migrating upstream. This stage of the stock is possibly the most difficult to catch and to sample. Such eels are too small to be caught by normal fishing gear. In addition, they tend to hide under stones and in weeds. To a certain degree one can take advantage of this behaviour by putting fashines or similar material in shallow water where natural hiding places are missing. Lifting the fashines over a gauze net may yield a good number of the hiding eels, but the transformation of the numbers thus caught to an estimate of stock size would be highly suspect and estimates of size distribution may be equally biased. Sampling is facilitated by the blocking of rivers by dams. For a long time eel-ladders have been used to help the eels to overcome such obstacles. Eel-ladders consist of such materials as grass ropes, straw, brushwood, wood shavings or nets, mounted on a plank or tube. The eels climb or crawl on or through the material. Additional by-pass currents may help to attract the eels to the actual ladder. Such ladders can be combined with facilities for catching the eels where climbing material ends before the top of the ladder in such a way that the eels are washed down through a gap or into a tube leading into a catching box. The more completely a dam or weir blocks the natural upstream migration, the greater the proportion of eels that are forced to use the eel-ladder and the better the quantitative and qualitative information on the annual recruitment of the stock. Behavioural studies on young eels have led to the development of brush-type elver traps/eel-ladders made of synthetic bristles (Köthke, 1964), which support the crawling behaviour of these fish much better than the traditional materials. Jens (1969) reports a fourfold increase of eels passing upriver compared to the fashine material used formerly. The author had the opportunity to develop the first nylon brush elver trap and tried it at a dam in the River Ems at Herbrum, where it successfully caught glass eels. Proposals were made within an advisory programme for the Electricity Supply Board of the Republic of Ireland to improve the management of the eel fishery within the Shannon River system, where the Board owns the fishery rights. Elver brush traps now have been introduced for dams and low head elver traps have been successfully developed (O'Leary, 1971). Brush ladders and traps have considerably increased the catch of young eels for stocking and are now the main type used. The diameter of bristles and the distance between them have to be adjusted according to the size of migrating eels. The trap itself can be kept small if it is mounted with flexible tubes for water supply, and the total gear can easily be transported and operated for sampling with little effort.
Where fish passes with adequate low current are present at dams or weirs, they are used by eels as well as by other fish. Sampling may be somewhat difficult; methods used are: trapping with fyke nets or with similar engines; closing the compartments with mesh screens and emptying the pass by closing off the water supply, electric counters for larger sized eels, etc.
After entering fresh waters the young eels migrating upstream tend to “settle” after some time or distance. Some eels remain in the estuaries, others continue their migration for several years each spring and summer and may climb up to the headwaters more than 1 000 km from the sea. The number of eels and frequently the percentage of males tend to decrease with distance from the sea, although in some areas as, for example, in the German Bight in the North Sea the coastal stock may have a greater number of females.
Settling of the eels is obviously relative as they always move before the final migration toward the spawning grounds. Thus, several types of migrations may be distinguished:
Long-distance upstream migration of young eels
Long-distance downstream migration of silver eels
Short-distance migrations of brown eels
Hibernation migration (in northern countries) from colder, shallow water to warmer, deeper areas in late autumn and inverse in spring (from coast to sea and back in coastal areas, from coast into river estuaries, and back downstream, and back in lowland rivers, from and to banks in lakes, etc.)
Feeding migrations (upstream with the rise of water in rainy periods and back, to spawning areas of other fish in spring and early summer, into weeds, etc.)
In the same area, eels may behave quite differently according to size, stage of maturation, food preference (broad- and sharp-nosed eels), tide (in estuaries), season, moon, rain, depth, etc. Thus, a sample (catch) cannot represent the stock in total but can select “behaviour groups” and this selection is also highly influenced by the type of gear. Furthermore, the same gear may select quite differently in different waters. Comparison of different sampling methods and cross checking with tagging experiments is recommended.
For adult eels sampling implies catching in order to obtain information rather than profit for other fish. Discussion of sampling methods means to discuss fishing gear in relation to the behaviour of fish. Fishing methods for eels have formed the subject of a Symposium on Eel Fishing Gear (EIFAC Technical Paper No. 14, 1971). The present state of knowledge of eel biology and management has been collected by Tesch (1973, 1977) and in 1977 a Symposium on Eel Research and Management was held jointly by EIFAC and ICES (in press). Everyone dealing with sampling of eels is recommended to make use of these documents.
Eels tend to hide in the substrate in holes or under such objects as stones, weeds, etc. This behaviour can be used to capture the fish. Tubes of bamboo, wood, metal or plastic are placed singly or in long lines on the bottom. The eels enter these hiding places and can be caught by closing the openings either by hand or by using funnels. Other materials used are bundles of twigs or netting from which eels have to be collected in dip nets as they escape during lifting. The efficiency of refuge traps depends on the density of natural hiding places and can be increased by baiting the traps (e.g., with small or minced fish, crabs or shrimps, mussels, worms, etc.). Selection according to size and type of eels is probable when refuge traps are used for warmwater brown eels in shallow waters, especially where the traps are baited.
Eel baskets are traditionally bottle-shaped with a 30-cm entrance and 1-2 funnels. They are made from wooden or plastic switches and are usually attached to anchored longlines, especially in rivers and tidal waters. Up to 50 traps can be mounted on one line. Recently, prefabricated traps have become available (Köthke, 1965). Eel baskets are fished either unbaited as refuge traps or, most commonly, baited as feeding traps. Their use is preferred for active, feeding brown eels in running water. Selection by size and type of eel is probable.
A large variety of types and sizes of fyke net are used worldwide (Brandt, 1972). The basic fyke usually consists of paired net baskets resembling those described above. Several sets are strung together in rows and are anchored in flowing waters at any depths. They are commonly baited. Winged fyke nets are used in more shallow waters either singly or in combination. They are fixed and stretched by stakes in stagnant water or in low-to-medium current. Winged fyke nets are mainly used unbaited. The eels and other fish are guided into the trap by means of the wings. This means that wings fyke nets are gears which eels may enter not completely voluntary, but may be forced to do so to a certain degree. Thus, a fyke net catch may include settled eels entering the trap more or less by chance as well as migrating brown eels or downstream-migrating silver eels. The size composition of a sample is affected by the mesh size and behaviour groups further influence the composition of the sample. Bank eels in rivers and lakes, downstream-migrating silver eels in rivers and up- or downstream-migrating brown eels in rivers, all have different characteristics. Fyke nets are also used in small outlet streams of lakes, especially to catch the autumn run of silver eels.
These are very large, complex, combinations of wings and traps of the fyke-net type. They are constructed in shallow water or at moderate depths and fixed to stakes or to anchored buoys. Pound nets are used in large lakes or on the coast; they are, for example, the main eel gear in the Baltic Sea. Wings guide the eels into chambers and fyke traps. The main catch is from silver eels in autumn. Pound net samples represent the run of silver eels, which may originate from far distant waters. So, in the Baltic Sea the eels caught in a pound net will represent mature eels from all coastal and inland waters flowing into the sea east of the fishing place and from the northern and southern parts of the Baltic.
Stow nets are large, conical nets which are kept in position by stakes or anchors either directly or by anchored boats or even using otterboard. They have one or two funnels and wodden hoops in the cod end may help to keep the net open, if the current is too slow. Usually the current of the river keeps the gear stretched and open. Stow nets resemble beam or otterboard trawls with the difference that while trawls are drawn through the water, in stow nets the gear is fixed and the water moves. In both cases a section of the water is filtered. The main difference between the two gears is that the fish drift or swim into the stow net whereas the trawl actively chases the fish. Apart from selection by mesh size, a stow net catch may include unbalanced proportions because of behavioural differences between groups of eels. Upstream migrating eels will be missed completely and eels settled locally will be under-represented. Stow nets mainly catch silver eels from upstream-located nursery grounds and may represent the local eels from the area fished. They catch and sample eels, which for one reason or another move downstream. Fluctuations in flow in summer, due to rainfall, may induce short or medium distance movements of the local stock. Consequently, stow net catches in summer are eels mainly drawn from local eels, which in fact may mean from a considerable part of the river. If a river is stocked with young eels caught in estuaries, as is frequently the case in German rivers, such eels (20–40 g) tend to escape downstream from their new environment. In the River Mosel, for example, these stocked eels appear in relatively high numbers in the stow net catches downstream for several days after stocking. Downstream-moving fish are not necessarily swimming, as they may be drifting. Thus a high percentage of fish weakened by disease or injury appraising in stow nets giving a false picture of the stock. Eels affected by red disease, cauliflower disease or damaged or killed by turbines, will be more likely to be included in stow net catches than in other gear catches.
Weirs are constructed in such a manner as to attempt to catch all downstream-migrating silver eels. The eels are filtered out of the whole flow by means of nets, wire cages or screens. The passage into the catching compartments can be narrowed by stone or wood barriers, fashines or other structures built out from the banks. Such eel weirs have been in operation for a long time in Canada (Day, 1948), Ireland (Frost, 1950) and in the outlet channels of the lagoon at Comacchio, Italy (D'Ancona, 1961). Catches from weirs are seasonally related to the timing of the silver eel migration in autumn. Eel traps in connection with water mills may be included in this group of permanent gears. In these all or part of the headwaters of a millpond flow over a sloping grill, from which the eels move or are washed down into a holding box.
Special seines for catching eels, as described by Walter (1910) are net walls with a trap in the centre, which are dragged by hand or motor toward the bank. The ground ropes are weighted to keep the net on the bottom. Seines are used in shallow-to-moderately deep waters and are limited to bottoms with no encumbances. Except for a possible selection by mesh size, the catches are representative of the area but not necessarily for the whole water. Unfishable grounds, which are too deep or covered with weeds, may hold concentrations of eels which vary in density per area or size groups.
Trawls are operated from motorized boats either as pair trawling or otterboard trawling. Trawling for eels is carried out at sea in the Baltic (Hoffmeister, 1970) and in the North Sea (Aker and Koops, 1976), but it is also done in lakes or slow-flowing rivers which are large and deep enough (Deelder, 1965; Steinberg, 1964). Depth is no limitation provided it is greater than about 5 m, but eels are usually less frequent at depths greater than 30 m. Bottom structure does, however, limit the use of trawls. Catch composition will fairly represent stocks, as most of the water area can be reached by this fishery.
This type of fishing with three-layer trammel nets is limited to shallow water. The net wall is placed along weed beds and by hitting the surface of the water the eels are driven toward the net where they become entangled.
Angling is practised by rod and line by sport fishermen or by longlines for both sport and commercial fisheries. The size and type of eels caught whether broad- or sharpnosed, are influenced by the hook size and the bait used and also change with the time of day, location, etc.
Spears are usually forbidden nowadays although special permits may be given for limited times and locations, especially in coastal regions. Spearing is mainly used to catch hibernating eels in winter at places where they congregate and in waters of less than 4 m depth. Size selection has to be considered.
The use of electricity for fishing is generally forbidden but special permission may be given if safety regulations are satisfactory. Electrification improves the efficiency of seines and trawls and is sometimes also used in connection with stow or fyke nets. However, electrofishing is usually taken to mean the catching of fish in an electric field between a dip net as the anode and a cathode. An adequate current forces any fish within a certain radius, usually a few metres, to swim toward the anode (electrotaxis) but they may even be stunned (electronarcosis). Fish of different form and size react differently to the current. This type of electrofishing is limited to shallow waters. Clear water gives better results, as stunned eels remain on or sink to the bottom and may easily be overlooked. Electrofishing is especially useful in streams and small-to-medium sized rivers and at the banks of lakes and large rivers. In the right type of water, electrofishing is probably the best method to get samples with little effort in a short time; the efficiency of the gear can easily be checked by fishing the same area two or more times.
Permission is rarely given for the use of toxins for killing or stunning eels. In any case, the use of poisons is not recommended for eels because they rarely drift to the surface and at the bottom most of them will be either overlooked or remain hidden in caves or weeds.
Reviewing the methods available for catching eels, we may distinguish two groups of samples:
Samples taken from commercial catches which are themselves samples of the stock. Catches may be biased considerably by several of the reasons mentioned above, i.e., size, gear or location related. Landings may be further biased as fishermen usually return small eels or eels unwanted for other reasons, for instance, diseased or damaged fish. Eels which are kept alive between catching and landing may become infected with diseases, especially red disease, and will lose weight. They also cannot be used for feeding studies. When taking sub-samples from catches, one has to be aware that it is not at all easy to obtain representative samples from the catch.
Experimental catches are usually limited to smaller gears such as baskets, small fyke nets, electrofishing or portable elver traps, which are easily handled.
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